These days required are increased integration of electronic devices and increased operation speed thereof, and for satisfying these requirements, the development of micropatterning technology is remarkable. As micropatterning technology, known is an immersion exposure technique (for example, Patent Reference 1).
The immersion exposure apparatus described in Patent Reference 1 comprises at least a lighting optical system including a photoexposure source of ArF excimer laser, a reticle (mask), a projection optical system, a liquid feeding unit and a liquid recovering unit. In the immersion exposure apparatus, the space between the wafer surface and the optical element on the wafer side of the projection optical system is filled with a liquid while the reticle pattern image is transferred onto the wafer.
The optical element comprises a liquid, of which the refractive index to the light at a wavelength of 193 nm from the ArF excimer laser is within a range of from 1.60 to 1.66, a substrate (lens) of which the refractive index to the light at a wavelength of 193 nm is within a range of from 2.10 to 2.30, and an antireflection film formed on the surface of the substrate to be in contact with the liquid. As the liquid, used is decalin (C10H18); as the substrate, used are garnet (lutetium aluminium garnet [Lu3Al5O12:LuAG], germanate, etc.] and spinel ceramic (Mg2Al2O4, etc.); and as the antireflection film, used is a laminate film of a metal oxide layer and a fluoride layer. Having the constitution, the light from the ArF excimer laser is prevented from reflecting between the liquid and the substrate, therefore attaining a high-resolution immersion exposure apparatus.
On the other hand, it is known that the resolution of the immersion exposure apparatus depends on the material having the smallest refractive index among the refractive index of the resist to be arranged on the wafer, the substrate (lens) of the optical element, and the liquid. A resist having a refractive index of more than 1.7 and a liquid having a refractive index of more than 1.6 have been developed; and a substrate having a refractive index of more than 1.7 is needed.
Patent Reference 1 describes use of WAG or the like as the substrate having a refractive index of from 2.1 to 2.30, in which, however, the transmittance of the substrate for the light from the photoexposure source (at a wavelength of 193 nm in Patent Reference 1) is not enough and therefore use of an antireflection film for preventing reflectivity depression is needed. For providing such an antireflection film, employed is a known physical or chemical vapor deposition method, which, however, makes the process complicated and increases the production cost. Accordingly, a substrate not requiring an antireflection film is desired.    Patent Reference 1: JP-A 2008-113004